1. Field of the Invention
The present invention relates to a current detector for detecting a current in a main circuit of a power switching apparatus and to a power switching apparatus having the function of protecting an apparatus or a device under a load due to an overcurrent.
2. Description of the Related Art
A power switching apparatus is used to open or close an electric path between a single- or three-phase power source and an apparatus or a device under a load as a fixed contact and a movable contact are made to open or close by driving an iron core and the movable contact leading thereto by energization of an electromagnetic coil.
FIG. 9 is a partially sectional view of a conventional power switching apparatus shown in Japanese Patent No. 1,751,690, and FIG. 10 is a partially sectional front view of the power switching apparatus.
In FIG. 9, a base 1 and a mounting base 2, which are formed of an insulating material, constitute a casing for fixing and holding internal component parts. An operating-coil driving circuit board 6 has a circuit for converting a signal supplied from an external power source to a coil driving signal. An operating coil 5 drives an iron core through a magnetic field generated by the coil driving signal. A fixed core 4 and a movable core 3 form magnetic paths for the magnetic field generated by the operating coil 5, and the movable core 3 is attracted to the fixed core 4 by the electromagnetic force acting in the gap between the fixed core 4 and the movable core 3.
A movable contact 9, which is connected to the movable core 3, comes into contact with a fixed contact 8 through the action of the movable core 3. As a result, a fixed contact point 11 and a movable contact point 12 attached to the contacts come into contact with each other, thereby making the electric path.
When the operating-coil driving signal is off, the driving force of the operating coil 5 is not present, and the movable core 3 and the fixed core 4 are separated by the force of a tripping spring 7, so that the fixed contact point 11 and the movable contact point 12 are separated, thereby breaking the electric path.
Incidentally, a terminal screw 10 connects an electric wire from an external circuit to the fixed contact 8.
FIG. 11 shows the structure of a conventional current transformer type current detector, and FIG. 12 shows the structure of a conventional Hall-effect type current detector.
In the drawings, a conducting portion 13 and a terminal screw 14 are for allowing the current sensor to be energized by the current of the apparatus or device under the load, a current transformer 15 is for detecting the current at the conducting portion 13, and an output line 16 outputs the current from a secondary winding of the current transformer 15.
A magnetic core 17 is formed of a magnetic material disposed around the conducting portion 13, and allows the magnetic flux generated by the current flowing across the conducting portion 13 to flow therethrough. The magnetic field is detected by a Hall-effect device 18, and is outputted as a current signal.
With the conventional current detectors, the current is detected by the through type current transformer or by the magnetic core having a gap and the Hall-effect sensor disposed in the gap. In either method, it is necessary to form the closed loop in which the core of the current transformer or the magnetic core serving as a passage for the magnetic flux is disposed around the conducting portion. As a result, the magnetic core becomes larger than the current sensor, so that the current detector is difficult to be built into a power switching apparatus.
In addition, since it is necessary to provide the magnetic core around the conductive portion, it is necessary to use a small-diameter conducting portion and pass it through the magnetic core during the assembly into a product. Hence, there is a problem in that after the terminal screw is fitted, the conducting portion cannot be built into the product.
Furthermore, in an area where the working current is large, the conducting portion becomes large, so that the magnetic core is also bound to be large in shape. Additionally, it is necessary to make the sectional area of the magnetic core large to prevent the magnetic saturation in the large current area, with the result that the magnetic core tends to become large, and higher cost of the current sensor portion is entailed.
In addition, the conventional current detector is difficult to be built into the power switching apparatus, and it has been difficult for the power switching apparatus to be provided with the functions of protecting the apparatus or device under the load, monitoring the operation of the apparatus or device under the load, and detecting a fault of the power switching apparatus.